A color filter, an important part of a liquid crystal display panel, must satisfy various requirements. In particular, the heat resistance and chemical resistance of a color filter are of importance in the production of super-twisted nematic (STN) or ferroelectric liquid crystal (FLC) display panels, in which a transparent conductive layer is to be formed on the color filter and further processed into a transparent electrode.
Color liquid crystal display panels are generally prepared by forming a multicolor image layer on a transparent glass substrate. The multicolor image layer generally comprises red, green, and blue pixels with alternately matrix arrangement. Though a size of the pixel is dependent on purpose for use, rectangular pixels, one side of which has a length of about from 10 to 100 .mu.m, and another side of which has a length of about from 50 to 400 .mu.m, is generally used. A shape of the pixels is not restricted to rectangle, and a pixel having a desired shape may be used according to purpose for use. In some cases, a multicolor image layer having black pixels or a light-shielding black matrix between or on the border of the pixels may be used.
A protective layer is formed on the multicolor image layer to obtain a color filter. Further, a transparent conductive layer is usually formed on the protective layer by sputtering indium-tin oxide (ITO). For producing an STN type or FLC display panel, the transparent conductive layer is processed into a transparent electrode by photolithography to obtain a color filter having an electrode. An orientation film is then provided thereon. The resulting substrate with a color filter is then assembled with another transparent substrate having a transparent electrode and an orientation film at a given gap therebetween by inserting spacers to produce a cell, and a liquid crystal is sealed into the cell.
Throughout the above-mentioned series of operations, the protective layer on the multicolor image layer should fulfill the following five requirements. (1) It should assure the flatness of the layer. (2) It should have sufficient hardness to ensure that the spacer does not sink into the cell gap and to ensure the presence of the prescribed cell gap. (3) It should be resistant to the various chemicals used in the photolithographic processing of the transparent conductive layer, such as the solvent of a photoresist, an acidic etchant, an alkaline resist stripper, and the solvent of a coating composition for an orientation film. (4) It should be transparent in the visible light region and be free from cloudiness or turbidity. (5) It should have excellent adhesion not only to the multicolor image layer but to the substrate, in case it is formed directly on the substrate in the absence of a multicolor image layer.
To meet these requirements, it has been proposed to use such a thermosetting resin (or a composition) as a composition comprising an epoxy compound and a polycarboxylic acid or an anhydride thereof as disclosed in JP-A-60-216307 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a nylon resin as described in JP-A-60-244932, and a composition comprising a melamine compound and an epoxy resin as described in JP-A-63-131103. A protective layer made of these thermosetting resins practically satisfies requirements (1) to (4) above.
On the other hand, there has been a demand that the protective layer on the non-image area should be removable from the transparent substrate, and the protective layer on a scribe line should be removable as described in JP-A-57-42009, JP-A-1-130103 and JP-A-1-134306. In this regard, it is difficult to selectively form a protective layer with good precision using the above-mentioned thermosetting resins.
Moreover, these thermosetting resins generally comprise a combination of an epoxy resin and a curing agent. These two reactive components, if allowed to stand in a mixed state, undergo a reaction with time which results in an increased viscosity. A coating composition having such an increased viscosity not only has a short pot life but fails to provide a coating film having a uniform and precise thickness.
In order to overcome this problem, it has been suggested that a photosensitive resin be used as the material of the protective layer, which cures on exposure to light and which can be developed to remove the unnecessary part.
Known photosensitive resins which have been proposed for use as the protective layer on the multicolor image layer include the ultraviolet-curing resin disclosed in JP-A-57-42009 and JP-A-60-244932, the vinylcarbonyl-containing polymer described in JP-A-59-7317, the photosensitive resin comprising polyvinyl alcohol and a photosensitive agent described in JP-A-59-184325, the rubber resins of JP-A-60-42704, and a photosensitive resin composition having the same component, except not containing pigment, as the photosensitive resin composition having a dispersed pigment, proposed for use as a colored layer as disclosed in JP-A-2-191901.
It is most desirable to form a protective layer by using a photosensitive resin which cures on exposure to light and can be developed with an alkali aqueous solution, as it is advantageous to health and environmental protection, and, after curing, can be rendered more alkali-resistant by heating (post-heating). A known method is that using the compositions described in JP-A-3-126950, JP-A-52-132091, and JP-B-4-20923 (the term "JP-B" as used herein means an "examined published Japanese patent application").
However, when this method is applied in the form of a coating film, these photosensitive compositions are still unsatisfactory for the formation of a protective layer in terms of developability, non-tackiness, alkali-resistance after post-heating, solvent resistance, transparency, and adhesion to a substrate. It is possible to form a photopolymerizable layer comprising such a photosensitive composition on a temporary substrate, which can then be transferred to a permanent substrate. However, the photopolymerizable layer made with a conventional photosensitive composition has insufficient transfer properties.
Further, where a transparent conductive layer is formed on a conventional protective layer, the protective layer, though resistant to the aforesaid various chemicals (a solvent of a photoresist, an acidic etchant, an alkaline resist stripper, and a solvent of a coating composition for an orientation film), causes the transparent conductive layer to crack.